For WP, use Office lens
Start the app, and from the menu (bottom right, 3 dots etc) import the jpeg from your photo library
Once open, proceed to save the file as pdf.
The file is store at store within the app by default (file:///C:/Data/Users/DefApps/AppData/Local/Packages/Microsoft.OfficeLens_.. ... *.pdf) in the phone memory, waiting to be uploaded to OneDrive

I found out I had to open that file (on Edge) again, and then save it to any location I want.

The neat thing about Office Lens is I can edit the jpeg, say a photo of a A4 size document taken a skewed angle, crop it along the edges. The resultant saved pdf (or jpeg) is perfectly rectangular.